I'm not familiar with the sound quality of the recordings you can take with an iPhone, but I would say for voice you need a condenser mic like the aforementioned Blue Snowball or Blue Yeti. Condenser mics are really sensitive and capture almost any sound in great detail.
If you just want it for convenience, I believe a lot of these types of microphones come with USB connectivity and possibly also come with desktop stands. Most other condenser mics use an XLR connection, which will require an audio interface to connect to your PC. All condenser mics will also need power to produce a strong enough signal to record. Most audio interfaces can supply power through the cable, but some mics also have room for batteries.(Power is supplied via USB for such models, so you won't have to worry about batteries).
I use the Audio Technica AT2035 for vocal recording, through an Alesis io4 interface and/or Line6 PodHD 500X (I have to use the io4 to power the mic, and the Pod is for effects like compression and reverb).
The cheapest version of this mic is the AT2020, which clocks in about £75 (probably about $100?). I think Audio Technica sells USB versions too, so you might want to look in to that.
If you are going to be close to your mic, you might want to consider a pop shield. They're quite cheap and they let you get super close for some really 'dry' recording. Getting too close increases the bass quite a lot, so a pop shield can also be used as a gauge to keep your mouth at a minimum distance.
I hope this helps; there are hundreds of potential microphone models out there, and quite a few are affordable for someone who isn't a wealthy audiophile or studio engineer!